Vibrometers can use passive or active means to illuminate a target and detect the target vibration by observing the phase modulation of the illumination scattered by the target relative to some phase reference. Emissions from the target may also be used in this way, either in response to illumination, or self-generated due to some physical property of the target. Optical, RF, or acoustic means can be employed, but typically optical means are used because the beams can easily be shaped and directed with precision, and the small wavelength leads to high phase modulation sensitivity for small target vibrations.
Typical vibrometers are subject to many signal degradations. Primary among these are relative motion between the vibrometer and the target (platform motion) that causes a Doppler shift of the frequency of the signal return, and speckle noise that can cause rapid dropouts due to interference between returns from different scattering elements of the illuminated target surface. In addition, the vibrometer can use a heterodyne architecture where the reference phase is obtained from a signal that is intentionally displaced in frequency from the probe illumination signal, and/or a homodyne architecture where the reference phase is derived from the transmitted illumination signal. The latter architecture has the advantage of lower cost, greater ruggedness potential, and simplicity, but has the disadvantage that there is an ambiguity in phase when there is no platform motion (i.e. at zero platform velocity, it is not apparent from the signal whether the vibrating surface is advancing or retreating relative to the vibrometer) that can lead to harmonic distortions.
Further, for either architecture, there is a limitation on the unambiguous amplitude that can be measured by phase detection alone because the transcendental functions that compose the motion are periodic on 2π, leading to a folding of the phase retrieval space. The latter can be easily addressed for a single frequency vibration signal by standard unwrapping techniques, but in the general case, the vibration is more complex with many different frequency elements each having a uniquely varying phase.